| 04 Jun 2026
Seasonal stratification regulates carbon allocation between particulate and dissolved pathways in the Gulf of Aqaba
Abstract. Water column stratification exerts fundamental control on microbial carbon cycling in oligotrophic areas of the ocean, yet its impact on the partitioning and fate of newly fixed carbon remains insufficiently resolved. Here, we investigated carbon fluxes in the Northern Red Sea, Gulf of Aqaba from 5 cruises conducted during the stratified period. We measured 14C-based measurements of primary production partitioned into particulate (>0.7 µm; PPPOC) and dissolved (<0.7 µm; PPDOC) fractions, bacterial production (BP), community and bacterial respiration across the euphotic zone (0–100 m). As stratification intensifies and nutrient supply from depth diminishes, depth-integrated PPPOC declined from 1.26 to 0.35 g C m-2 d-1 while the relative contribution of dissolved carbon pathways increases. The fraction of newly fixed carbon released as dissolved organic carbon (extracellular release; PER) increased from 2.5 % to >7 % of total PP, indicating that a larger fraction of photosynthetically fixed carbon was released into the dissolved C pool. PPDOC (0.02–0.03 g C m-2 d-1) was positively correlated with BP (0.08–0.16 g C m-2 d-1), suggesting that recently released dissolved substrates contribute to sustaining heterotrophic microbial activity. Despite declining primary production, BR remained substantial (0.23–0.52 g C m-2 d-1), resulting in low to moderate bacterial growth efficiency (13–35 %) and indicating that most processed carbon was respired rather than incorporated into biomass. These findings indicate that summer stratification enhances the relative importance of dissolved carbon release and microbial recycling, thereby reducing the efficiency of carbon transfer to depth in the Gulf of Aqaba and likely other oligotrophic systems.
This manuscript presents a valuable dataset on microbial carbon cycling during the stratified season in the Gulf of Aqaba. By combining measurements of particulate and dissolved primary production, bacterial production, community respiration, and ancillary environmental parameters, the authors provide a useful perspective on how seasonal stratification may alter carbon allocation between particulate and dissolved pathways in an oligotrophic marine system.
The study addresses an important question and the dataset is unique. I found the manuscript generally well organized and suitable for publication after revision. However, several aspects of the analyses and interpretations require clarification and some conclusions should be more cautiously framed.
1. One of the main conclusions of the manuscript is that increasing stratification shifts carbon cycling toward dissolved pathways and enhances microbial recycling. Since both the percentage of PER and BGE were calculated, it would be informative to directly examine whether these two quantities are related. For example, is BGE higher when a larger fraction of newly fixed carbon is released as dissolved organic carbon? A simple correlation analysis between PER and BGE could provide additional support for the proposed coupling between dissolved carbon release and bacterial carbon utilization.
2. The manuscript interprets the observed BGE values as characteristic of strongly recycling oligotrophic systems. While this interpretation is reasonable, recent studies from other oligotrophic environments have reported comparatively high BGE values despite nutrient limitation. For example, recent work conducted in the Taiwan Strait found relatively elevated BGE under oligotrophic conditions (Liu et al., 2026; JGR-Oceans). It would be useful to discuss these contrasting observations. In particular, what factors might explain why oligotrophic systems sometimes exhibit low BGE and sometimes relatively high BGE? A brief discussion placing the Gulf of Aqaba observations within this broader context would enhance the manuscript.
3. Some statements need to be clarified.
Lines 352-356: I found this interpretation difficult to follow. If chlorophyll biomass becomes increasingly concentrated in the DCM, it does not necessarily imply “low standing stocks.” Could lower bacterial consumption at DCM explain this pattern?
The authors should clarify what they mean by “maintaining low standing stocks but operating at reduced photosynthetic efficiency” and explain the mechanistic basis for this decoupling more explicitly.
Lines 381-386: The contrast with Devresse et al. (2022) is currently confusing. Earlier in the paragraph, semi-labile DOC is presented as a reservoir that buffers temporal mismatches between production and consumption. However, in the ETNA example, semi-labile DOC appears to support a production-driven carbon cycle. Please clarify whether the semi-labile DOC in these two cases represents accumulated DOC pools versus freshly produced DOC associated with contemporaneous primary production.
Lines 418-420: If nutrients are rapidly taken up by microorganisms, biological processes are clearly influencing their distributions.
Lines 424-427: Earlier results indicate that PER values are also relatively high near the DCM. Could the pronounced DCM observed during stratification primarily reflect photoacclimation rather than true biomass accumulation? Therefore, it is not obvious why carbon fixed at the DCM should preferentially contribute to biomass accumulation and export rather than dissolved pathways.
Minor comments
Please define all abbreviations when they first appear in the manuscript. For example, DOC and silicic acid [Si(OH)4] should be introduced with their full names before abbreviations are used.
Lines 80–88: The knowledge gap regarding the partitioning of newly fixed carbon and its connection to heterotrophic metabolism is introduced twice in slightly different wording. Streamlining this section would improve readability and sharpen the study objectives.
Line 251: The phrase “in June–September to (Figure 2B)” appears incomplete and should be corrected.
Line 256: “in the area” are rather vague. Please specify this region.